Introduce Substitution class and Supertype class.

test/type_substitute_bounds_test.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 library kernel.type_substitute_bounds_test;
 
 import 'package:kernel/kernel.dart';
 import 'package:kernel/type_algebra.dart';
 import 'package:test/test.dart';
 import 'type_parser.dart';
 
@@ skipping 54 matching lines @@
     test('$testCase', () {
       var environment = new LazyTypeEnvironment();
       var type = environment.parse(testCase.type);
       var upperBounds = <TypeParameter, DartType>{};
       var lowerBounds = <TypeParameter, DartType>{};
       testCase.bounds.forEach((String name, TypeBound bounds) {
         var parameter = environment.getTypeParameter(name);
         upperBounds[parameter] = environment.parse(bounds.upper);
         lowerBounds[parameter] = environment.parse(bounds.lower);
       });
-      var substituted = substituteBounds(type, upperBounds, lowerBounds);
+      var substituted = Substitution
+          .fromUpperAndLowerBounds(upperBounds, lowerBounds)
+          .substituteType(type);
       var expected = environment.parse(testCase.expected);
       if (substituted != expected) {
         fail('Expected `$expected` but got `$substituted`');
       }
     });
   }
 }